The HP-45 was the second scientific calculator introduced by Hewlett-Packard, adding to the features of the HP-35. Especially noteworthy was its pioneering addition of a shift key that gave the other keys alternate functions. It also contained an Easter egg that allowed users to access a inaccurate stopwatch mode. An accurate version of the stopwatch mode was officially featured in the 1975 successor of the HP-45, the HP-55.